The people living on either side of the Ugandan-Rwandan border at Katuna never much considered the boundary: Children crossed for school, workers moved freely and trade thrived.

That harmony evaporated in February when Rwanda abruptly closed the crossing, with queues of cargo trucks and thronging merchants turned back as soldiers from both armies marshalled along the forest-clad border.

The blockade is a result of the worsening animosity between Rwanda's Paul Kagame and Uganda's Yoweri Museveni, once close allies who backed each other into power, but whose relationship has turned deeply hostile.

The distrust between the presidents has burst into the open in recent months, with the pair trading accusations of espionage, political assassinations and meddling in each other's backyards.

The spat risks dragging in their neighbours, threatening economic integration and regional stability in an already conflict-prone swathe of the continent.

The standoff escalated dramatically in March when Rwanda publicly accused Uganda of abducting its citizens and supporting rebels bent on overthrowing the government.

Museveni - who has admitted meeting, but not endorsing, anti-Kagame rebels - harbours his own suspicions about his erstwhile ally. His officials have accused Rwandans in Uganda of spying, and some have been detained by military courts or deported.

"What is wrong is for Rwandan agents to try and operate behind the government of Uganda," Museveni wrote to Kagame in March.

The following month, Kagame threatened perceived enemies as he digressed during a speech marking the 25th anniversary of Rwanda's genocide.

"Those who think we have not seen enough of a mess, and want to mess with us, whether from here or from outside, I want to say: We will mess up with them big time," Kagame declared to applause, before an audience peppered with foreign dignitaries.

Biting blockade

The sabre-rattling has not turned violent to date.

Ugandan troops are busy at Katuna, the border town, where earth-movers can be seen clearing a road to a hilltop military base. Rwandan forces, meanwhile, are patrolling their side.

But the interruption to trade is taking its toll.

Food prices have jumped in Rwanda, which relies heavily on imports from its larger northern neighbour. The blockade has also severed Uganda's land access to export markets in Democratic Republic of Congo (DRC) and Burundi.

And for communities straddling the border, the leaders' row has proved disastrous.

"I have lived and worked here most of my life... business has never been this bad," said Philemon Mugasha, a Ugandan clearing agent at Katuna, surveying the deserted streets usually buzzing day and night with money changers, food vendors, truck drivers and prostitutes.

Rwandans are taking great risks to cross the border under the cover of darkness. In May, a bean trader was shot in the arm by Rwandan troops as he attempted to enter Uganda.

"If they see us, they will arrest us and beat us for defying their order... it is dangerous," said one Rwandan worker who crossed illegally at Katuna to find work, and could not return.

Friends to foes

For decades, armies commanded by Kagame and Museveni fought shoulder to shoulder in East Africa, thrusting their generals into high office, and overthrowing the DRC's rapacious dictator Mobutu Sese Seko in 1997.

Kagame was appointed Uganda's military intelligence chief after his army of Rwandan exiles helped Museveni take power in 1986. Museveni later returned the favour, backing Kagame's invasion of Rwanda that ended the 1994 genocide and secured his grip on the country.

But by the turn of the century, the romance was over. A festering unease between their armies during the DRC campaign erupted into open warfare in the city of Kisangani, leaving hundreds dead between 1999 and 2000.

The relationship never recovered. But despite the bad blood, Rwanda and Uganda maintained full diplomatic ties, and troops stayed in their barracks.

The neighbours both enjoyed robust economic growth and strong relations with the West. Rwanda is a major contributor of peacekeepers, and Ugandan soldiers are fighting terrorism in Somalia.

Kagame's enemies

While Kagame did not name his enemies in his recent speech, shadowy rebel groups seeking to oust him have proliferated in the region.

Rwanda has boosted security in its southwest region bordering Burundi - with whom it has long had sour relations - and the DRC.

There have been several bloody incursions into Rwanda by one anti-Kagame rebel group known as the National Liberation Front (FLN), based in the DRC.

Western allies have advised their citizens to steer clear as the attacks target a region popular among foreign tourists.

The UN has also reported that Kayumba Nyamwasa, a Kagame foe and former military chief who runs the Rwandan National Congress, could be raising a rebel army in the DRC.

Kagame sees Museveni's invisible hand in the move against him.

"This is a contest to determine who between them is the kingmaker in the region," said Christopher Kayumba, a political analyst in Kigali.

War seems unlikely, experts say. But the brinkmanship is inflicting casualties each day it drags on.

"They say when two elephants fight, it is the grass that suffers. We are suffering," said Mugasha, the customs agents at Katuna.